Effect of crystal orientation on fatigue crack initiation life in pure aluminum single crystals

Abstract The crystal orientation dependence of fatigue crack initiation life was studied on 99.99 % pure aluminum single crystals. The fatigue crack initiation life is not ruled only by Primary slip system, but also affected by the second, the third and the fourth slip systems. Fatigue damage formation is accelerated in the crystal orientation which has equally large Schmid factor both on Primary and Critical slip planes, but it is restrained as Schmid factor on Conjugate slip plane increases. X-ray micro-beam back reflection Laue observation showed that the subgrain structure was remarkably developed. The configuration of crystallographic fatigue damage depends on the crystal orientation. The secondary slip system plays an important role in it. The total misorientation observed at the fatigue crack initiation takes about 5.5x10-2rad, not depending on the crystal orientation and the shear stress amplitude. This suggests that the fatigue crack is initiated when the dislocation density at the subgrain boundary reaches a certain value.